This invention relates to projection displays and is a way of projecting an image through a light guide with optimal efficiency and minimal margin.
Video projectors produce big moving images at low cost. An inexpensive way of making a television is, as shown in FIG. 1, to point a projector 1 via a mirror 3 onto the rear of a diffusive screen 5. This form of projection television is, however, bulky and users prefer displays to be slim.
A slim projection display can be made according to the applicant's earlier WO 01/72037 by pointing a video projector into the thick end of a tapered light-guide. The principle is illustrated in FIG. 2; the rays entering the thick end 12 of a tapered-panel waveguide 10 via an inclined face bounce at progressively steeper angles until they exceed the critical angle and exit; a shallow ray (solid line) travels further before this happens and therefore exits further along the display (up, in the usual orientation). This is called the tapered-waveguide principle, though it could be brought about by GRIN techniques instead of a purely geometrical taper.
A problem is that, since the projector is much smaller in the lateral dimension than the panel, rays fan out from the point of injection, so the projected image will be V-shaped. Furthermore, the projected image will be broken into bands: each band contains all the rays that undergo a given number of reflections, while the set of rays which have undergone one pair of reflections more or less than rays exiting in adjacent bands will be separated by a gap.
As explained in WO 01/72037, one can insert a transparent input slab of constant thickness between the projector and the tapered light-guide; this means that rays will have the opportunity to fan out before entering the tapered light-guide, so that the projected image becomes trapezoidal.
This is less objectionable than a V-shape but there is still significant keystone distortion. Moreover, viewers like images to fill the screen, so it is desirable to fold the input slab behind the tapered light-guide. This can be done with a pair of right-angled prisms spanning the width of the screen.
A ray entering the input slab at slightly less than the critical angle with respect to its faces undergoes many reflections in the slab but few in the tapered light-guide, whereas a ray entering at much less than the critical angle undergoes few reflections in the slab and many in the tapered light-guide. WO 03/013151 by the applicant explains how to shape the tapered light guide in order that the sum of reflections through the system is the same for rays at all angles of entry, so the projected image is no longer broken into bands. This is shown in FIG. 3, with the parallel-face slab indicated by the numeral 20. This shape of output waveguide is similar to a simple wedge with flat faces, tapering to an edge, whose length is approximately 1.5 times greater than that of the slab. However, one surface of the taper curves outwards slightly so that the thickness of the taper is 10% greater at the half-way point than if the surface were flat.
Projector lenses are less expensive if fan-out angles are small, so it makes sense for the slab to be no shorter than the tapered light-guide, in which case the latter is in effect truncated by a third, as indicated by dashed lines in FIG. 3. The truncated taper which results has a less steep angle of taper than if the light-guide tapered to a point over the same distance. If the angle of taper becomes less steep, then the final angle of intersection made by a ray as it leaves the system is closer to the critical angle and less of the ray is transmitted, which degrades system efficiency. Furthermore, a truncated taper is heavier than one which tapers to a point (edge) from the same starting thickness over the same distance.
While the surfaces of the slab and the tapered light-guide alone have little, if any, curvature, there is a kink at the point where slab meets taper and this degrades the projected image. One can replace this kink with a curve which smoothes the transition from slab to taper, as shown in FIG. 4, but one cannot fold a light-guide whose surfaces are curved. The folding prisms must therefore either be placed between slab and transition region, in which case the transition region becomes an unsightly blank margin beneath the projected image, or the folding prisms must be placed between transition region and wedge, in which case the transition region adds to the length of the slab. If the transition region is long, then the combined length of transition region and slab is such that they extend beyond the length of the wedge in an unsightly manner.
Tapered light-guides can also be used in reverse as explained by WO 02/45413 so that a camera pointed into the thick end captures an image of whatever is placed against the side of the light-guide; here the same problems with the transition region arise.